SPIN GAP IN A QUASI-ONE-DIMENSIONAL S=1 2 ANTIFERROMAGNET - CU-2(1,4-DIAZACLOHEPTANE)(2)CL-4/

Cu-2(1,4-diazacycloheptane)(2)Cl-4 contains double chains of spin-1/2 Cu2+ ions. We report ac susceptibility, specific heat, and inelastic n eutron-scattering measurements on this material. The magnetic suscepti bility chi(T) shows a rounded maximum at T = 8 K indicative of a low-d imensional antiferromagnet with no zero-field magnetic phase transitio n. We compare the chi(T) data with exact diagonalization results for v arious one-dimensional spin Hamiltonians and find excellent agreement for a spin ladder with intrarung coupling J(1) = 1.143(3) meV and two mutually frustrating interrung interactions J(2) = 0.21(3) meV and J(3 ) = 0.09(5) meV. The specific heat in zero field is exponentially acti vated with an activation energy Delta = 0.87(1) meV. A spin gap is als o found through inelastic neutron scattering on powder samples that id entify a band of magnetic excitations for 0.8<(h) over bar omega<1.5 m eV. Using sum rules we derive an expression for the dynamic spin-corre lation function associated with noninteracting propagating triplets in a spin ladder. The Van Hove singularities of such a model are not obs erved in our scattering data, indicating that magnetic excitations in Cu-2(1,4-diazacycloheptane)(2)Cl-4 are more complicated. For magnetic fields above H(cl)similar or equal to 7.2 T specific-heat data versus temperature show anomalies indicating a phase transition to an ordered state below T = 1 K.